Charge integrating bistable storage tube



United States Patent 3,325,673 CHARGE INTEGRATHNG BHSTABLE STORAGE TUBERobert H. Anderson, Portland, Ureg., assignor to Tektronix, lne.,Beaverton, Greg., a corporation of Oregon Filed Aug. 19, 1963, Ser. No.302,889 9 Ciaims. (Cl. 315-412) The subject matter of the presentinvention relates generally to electron discharge display devices and inparticular to cathode ray storage tubes which receive electrical inputsignals, store such signals for -an indefinite controllable time andreproduce such signals as visual images for direct viewing or aselectrical output signals.

The storage tubes of the present invention may be employed in a cathoderay oscilloscope for recording transient signals, in la radar or sonardisplay device, as a character writing tube and as a signal delay deviceto store electrical signals for .a controllable time before producing anelectrical output signal which is delayed with respect to such inputsignal. The present storage tube is an improvement over my previousstorage tubes disclosed in the following copending U.S-. patentsapplications: Ser. No. 180,457, now U.S. Patent 3,293,473, entitled,Electron Discharge Display Revice, filed Mar. 19, 1962; Ser. No.214,877, now US. Patent 3,214,631, entitled Storage Tube, filed Aug. 6,1962; and Ser. No. 245,716, now U.S. Patent 3,214,516, entitled,Electrical Readout for Storage Tube, led Dec. 19, 1962, of which thepresent application is a continuation in part.

Conventional storage tubes employ complex targets including dielectricmaterial deposited on a conducting wire mesh which together with othertargets are shown by A. V. Haeff in U.S. Patent No. 2,761,098, issuedAug. 28, 1956, and F. H. Harris, in US. Patent No. 2,839,679, issuedJune 17, 1958. The storage tube of the present invention is lesscomplicated and less expensive to manufacture than conventional storagetubes because it employs a storage target of such simpler construction.The conducting wire mesh of conventional storage tubes usually has thestorage dielectric deposited on one sid`e of the wires of such mesh sothat such target exerts grid control on flood electrons which passthrough apertures in such dielectric coated mesh to a separate phosphorviewing screen. Such tubes are often referred to in the art as the gridbarrier or transmission type of storage tubes. They are characterizedIby the storage of a charge image on the dielectric layer which coatsthe mesh, and phosphor mesh to the selective transmission of electronsthrough such viewing screen coated on a glass support plate spaced fromsuch mesh to produce a visible light image corresponding to such chargeimage. In contrast, the storage target of the present invention may bein the form of simply a thin integral layer of phosphor materialsupported on the face plate portion of the tube envelope over a targetelectrode which is a light transparent conductive coating. This phosphorlayer serves both of the two purposes of providing storage of the chargeimage `and production of the visible light image, which formerlyrequired two separate structures in the commercially manufacturedtransmission types of charge storage tubes.

The integral layer of phosphor forming the target of the presentinvention is a continuous uniform layer in its gross macroscopicstructure, but the individual phosphor particles which make up the layerare deposited so that they are not in good electrical contact with eachother, especially at the vacuum side of the target surface. This layerof phosphor has a somewhat porous or permeable structure which enablessecondary electrons emitted from one side of the layer to be transmittedthrough such layer and collected by the target electrode and results indesirable storage characteristics as described in my copendingapplication Ser. No. 180,457. The target layer of the present inventionis not continuous in its electrical conductivity in a directiontransverse to the tube axis. The term semicontinuous layer will 'be usedthroughout this description to refer to the structure and electricalproperties of the combined viewing screen and storage dielectric layerdescribed herein. Such semicontinuous phosphor layer also has athickness within a critical range of thicknesses so that it may performthe functions of both charge image storage and light image emission.

ln a sequence of experimental tubes, increasing amounts of phosphor wereused, until it was found that there is an upper limit to the phosphorthickness which can be used and still have an adequate stable range ofstorage voltages. For P-l phosphor, it was found that inadequate stablerange of complete loss of storage generally occurs when more than about31/2 to 41/2 mg. per cm.2 is used. This upper limit of thickness isroughly 1/s to 1/2 of the thickness used in conventional cathode raytube P-l phosphor viewing screen. Layers of this thickness appear to theeye to be fairly dense, conventional continuous layers. However,displacement tests of the packing density of P-l phosphor layers showthat about half of the thickness of such layers is empty space, yandonly about half of the volume of such layers is actually occupied by P-lphosphor particles. The porosity of the resulting layers, and the use ofthin layers results in the semicontinuous target surface of thisinvention. This layer is sufliciently discontinuous to prevent spreadingof stored images, which appears to be a necessary feature for adequatebistable storage stability.

A mesh electrode may be mounted adjacent to but spaced from the surfaceof the storage target between such target and the sources of electronswhich bombard the target so that positive ions, produced when suchelectrons ionize the residual gas in the tube, are repelled away fromthe target to reduce the damage to the phosphor layer caused by ionbombardment. The mesh electrode is connected to a positive D.C. voltageand establishes a substantially uniform potential gradient adjacent therear surface of the phosphor layer which enables more uniform4distribution of the flood electrons emitted by the viewing or flood gunin such tube and used to maintain the charge image on such phosphorlayer. This prevents bright `areas from appearing in the backgroundillumination of the storage target which were caused by the deflectionof such flood electrons by the charge on adjacent phosphor areas ofdifferent potential due to a type of coplanar grid effect. It should benoted that the mesh electrode may also function to collect some of thesecondary electrons emitted by the storage target.

It has been found that the storage characteristics of a storage tube canbe improved by effectively turning off the -iiood gun within such tubeduring the writing of a charge image on the storage target of such tubeso that the low velocity flood electrons do not bombard the target atthis time. This technique may be employed to increase the writing rateof the tube in order to store the charge image of a high speed transientsignal which previously could not be stored because the flood electronsact to oppose writing The ood electrons drive the potential of thetarget areas, initial-ly charged to a voltage below the first cross-overvoltage on the secondary emission curve of the phosphor storagedielectric, down toward the voltage of the flood gun cathode. Thus, thepotential of the charge image of a transient signal is often preventedfrom building up to the first cross-over voltage, which is the minimumvoltage necessary for storage, by the opposing action of the floodelectrons. This opposing s action may be eliminated by preventing theood electrous from striking the storage target during the production ofthe charge image on such target by the bombardment of such target withthe high velocity writing electrons, until the potential of such chargeimage exceeds the first cross-over voltage.

A simi-lar method of operation is effective for the storage of lowlevel, fast rise time or high frequency repetitive signals in which theiiood electrons are prevented from striking the storage target duringthe writing of several consecutive signals. Since these signals have thesame wave form, the potentials of the superimposed charge imagescorresponding to such signals are added together, When the totalpotential of the charge images is greater than the first cross-overvoltage, the ood electrons are allowed to bombard the storage target tostore such charge image for an indefinite controllable time. This chargeimage integration involves the charging of the target capacitance indiscrete amounts of voltage applied in response to each successivesignal.

Another improved method of operation of the direct view storage tube ofthe present invention increases the image contrast of the light imageemitted by the phosphor target layer and may be accomplished by applyingcontrast enhancement pulses to the flood gun cathode or to the targetelectrode layer of the storage target. These contrast enhancement pulsesmay be rectangular pulses having the proper polarity to increase thevelocity of the flood electrons bombarding the storage target a smallamount for approximately 50% of the flood gun duty cycle. In addition,both the writing speed and the erase speed of the tube may be increasedsomewhat by shifting the operating level of the storage target electrodeto a more positive voltage during writing and to a more negative voltageduring erasing, such as by pulsing the target electrode layer.

Briefly, one embodiment of the storage tube of the present invention mayinclude an evacuated envelope, a writing beam producing electron gunstructure having horizontal and vertical deection plates, a flood beamproducing electron gun structure, a storage target supported on theinterior of the face plate portion of the envelope, and a mesh electrodesupported between the electron guns and such storage target. The storagetarget may be in the form of a light transparent substrate body ofelectrical insu-lative material, such as a flat face plate portion ofthe envelope, with a light transparent iilm of electrically conductivematerial supported n one side of such substrate body, and a chargestorage dielectric layer of secondary emissive phosphor materialsupported on such substrate body over such conductive iilm. The storagedielectric layer may be a semicontinuous layer of phosphor having athickness that is within a critical range of thicknesses over which suchphosphor material will store an electrical charge image for anindefinite controllable time in the tube and will emit a visible lightimage corresponding to such charge image, when bombarded by electrons.The envelope may have a rectangular funnel portion of ceramic material`with a flat rectangular face plate of glass sealed thereto. A pluralityof axially spaced conducting wall coatings may be applied to theinterior of the funnel portion of the envelope and provided withelectrical connections to the exterior of such envelope so that suchcoatings may function as electrodes to focus, collimate and collect theprimary electrons generated by the flood gun structure and, in somecases, to collect the secondary electrons emitted by the storage target.The mesh electrode is supported over the phosphor layer of the storagetarget spaced from such phosphor layer and may be connected to one ofthe wall coating electrodes to operate at the same voltage as such wallcoating electrode. The wall coating electrodes along with the writinggun, the flood guns and the conductive film of the storage target areall connected to suitable sources of electrical potential so that thestorage tube of the present invention functions as a i bistable storagetube in that the target voltage on every area element of the surface ofthe storage layer tends to be held at one of two stable states ofelectrical potential.

It is therefore one object of the present invention to provide animproved storage tube which is simple and inexpensive to manufacture andwhose structure is resistant to mechanical shock and vibration.

Another object of the invention is to provide an improved direct-viewingtype storage tube having a storage target of simplified construction inwhich a semicontinuous layer of phosphor material having a thicknesswithin a critical range of thickness is employed both as a storagedielectric for storing the electrical charge image without imagespreading and as a uorescent material for emitting the visible lightimage of such direct-viewing storage tube, and having a mesh electrodespaced adjacent such target to repel positive ions away from the targetand to more uniformly distribute the ood electrons over the phosphorlayer to provide substantially uniform background illumination.

A still further object of this invention is to provide an improvedmethod of operation of a direct-viewing bistable storage tube in whichthe contrast of the visible light image emitted therefrom issubstantially increased.

An additional object of the present invention is to provide an improvedmethod of operating a storage tube in which the low velocity electronsof the viewing gun in such tube are prevented from striking the storagetarget during the bombardment of such target with high velocityelectrons from the Writing gun of such tube, until the potential of thecharge image produced on the target exceeds the minimum voltagenecessary for storage.

Another object of this invention is to provide an improved method ofoperation of a bistable storage tube which increases the maximum writingspeed of the electron beam emitted by the writing electron gun in suchtube `which will result in storage of a charge image produced by suchelectron beam on a storage target in such tube.

A still further object of the present invention is to provide animproved method of operation of a bistable storage tube by which theerase speed of such tube is increased so that the charge image on thestorage target employed in such tube may be removed and such target maybe reconditioned for storing another such image in a shorter time.

Additional objects and advantages of the present invention will beapparent from the following detailed description of certain preferredembodiments of the present invention shown in the attached drawings ofwhich:

FIG. 1 is a side view of one embodiment of the storage tube of thepresent invention with portions broken away to show internal structure;

FIG. 2 is a front view of the storage tube shown in FIG. 1;

FIG. 3 is a fragmentary horizontal section View taken along line 3-3 ofFIG. l, showing, on an enlarged scale, one embodiment of the storagetarget which may be employed in the tube of FIGS. 1 to 3; and

FIG. 4 shows the storage and display characteristics of the phosphorstorage target of the present invention.

A preferred embodiment of the storage tube of the present invention isshown in FIGS. 1 to 3 to include an evacuated envelope having a tubularneck portion 10 of glass, a funnel-shaped body portion 12 of ceramicmaterial and a fiat rectangular face plate portion 14 of glass. Thisenvelope structure may be similar to that shown in the copending U.S.application Ser. No. 132,915, now U.S. Patent 3,207,936, entitledElectron Beam Display Device, filed by W. H. Wilbanks et al., on Aug.21, 1961. As stated in such copending application, the glass neckportion 10 may be sealed to the ceramic funnel portion 12 by a glassfrit seal 16 at the small end of such funnel portion While the glassface plate portion 14 may be attached to the larger end of such funnelportion by a similar glass frit seal 18. The neck portion of theenvelope may contain an electron gun structure including a writing gun17 and one or more flood guns 19. Electrical lead pins 2o may extendthrough the side of the neck portion 1d to the horizontal deflectionplates, the vertical deflection plates and the isolation shield of suchwriting gun and to the focusing electrode and isolation shield of suchflood guns through a flame seal which joins two tubular glass membersforming the neck portion of the envelope. Other electrical leads toelectrodes in the writing gun and the flood guns may extend through aseal (not shown) at the base end of the neck portion of the envelope andconnected to pins 22 in a plastic base 24 which is suitably secured tosuch end of envelope neck portion.

The direct viewing storage tube of the present invention may include asplit screen storage target 26 of the type described in my copendingapplication Ser. No. 214,877, now U.S. Patent 3,214,631, which may bepositioned and supported on the interior surface of face plate 14 andwill be ,described in greater detail with reference to FIGS. 2 and 3. Aplurality of separate electrodes may be provided as spaced wall coatingsof a conductive material, such as silver, tin oxide, aluminum orgraphite, on the interior surface of the funnel portion 12 of theenvelope. The first electrode wall coating 2S functions primarily as afocusing electrode for the flood electrons emitted from flood guns. Itis connected to a suitable source of electrical potential through afirst connector plug 30 which extends through a hole in funnel portion12 of the envelope. A second electrode wall coating 32 having a greaterlength than the first electrode 28 is spaced from such first electrodeand electrically connected to the exterior of such envelope by a secondconnector plug 34 so that it can also function as a focusing electrode.A third electrode wall coating 36 is provided on the interior surface offunnel portion 12 and spaced from the second electrode 32,. It ispositioned near the storage target 26 and functions primarily as afocusing and collimating electrode for the flood electrons so that suchelectrons are substantially uniformly distributed over the surface ofthe storage `target 26 and approach such target at approximately rightangles thereto. The third electrode wall coating 36 is also connected toa source of electrical potential by a third plug connector 38. A meshelectrode 40 is supported over the storage target 26 between the secondand third wall coatings spaced from such target. This mesh electrode maybe in the form of a metal wire screen or a foraminous plate of anextremely open mesh which may be connected to the second wall coating32. The primary function of the mesh electrode 40 is to repel positiveions of residual gas away from the storage target to prevent thedestruction of such target. However, the mesh electrodes also enablesmore uniform distribution of the flood electrons over the storage targetto eliminate bright areas in the background illumination of such target.

lt should be noted that a conventional resistive coating 44 of aquadagor similarly conductive material is provided on the interior of aportion of the envelope neck portion 10 and electrically connected tothe isolation shield of the writing gun contained in such neck portionso that it serves as an extension of the second anode in such writinggun. A more conductive coating 46 of silver or the like may be providedover the end of conductive coating 44 spaced from the end of the firstelectrode coating 28, in order to provide a more uniform electricalfield at the end of such conductive coating.

A graticule scale 48 is provided on the interior surface of the envelopeface plate portion 1d by applying lines of fused glass frit or whiteinsulator material thereto or by scribing notches therein as shown inFIGS. 2 and 3. This internal graticule may be illuminated by a suitablesource of light (not shown) positioned outside of the envelope as shownin the above-mentioned copending US. Patent 3,207,936, so that the lightis transmitted through the surrounding edge of face plate 14 to suchgraticule scale. Storage target 26 extends over the graticule scale d@on the interior surface of face plate 14 and, as described below, has apair of spaced upper and lower conducting layers extending to theexterior of the envelope which serve as target electrodes. An electricalconnection can be made to each of such target electrodes by spacedconnector coatings Sti and 51 of silver or the like on the exteriorsurface of the envelope over the glass seal 18.

One embodiment of the storage target 26 of the present invention isshown in FlGS. 2 and 3 to include two thin light transparent conductivelayers or films N2 and 103 coated in spaced insulated relationship onthe interior surface of glass face plate 1d. These transparentconductive films may be made of tin oxide formed from stannous chloride,or other suitable material, and are applied to the surface of the faceplate in any conventional manner over the graticule scale dit which maybe in the form of fused glass lines. The storage dielectric of storagetarget 26 is a thin semi-continuous layer of phosphor material 104i, forexample, P-l type phosphor having a chemical designation ofZn2SiO4zlA/1n, which is applied over the upper and lower conductivefilms 102 and 103 in any suitable manner, such as by water settling onthe film or the application of a preformed layer of phosphor to the filmby a decalcomania technique, to provide a thin porous integral layer ofphosphor having a substantially uniform thickness. Such thickness iswithin that range of thicknesses over which such phosphor material willstore a bistable charge image for an indefinite controllable time, withno substantial spreading or migration of the charge image and will emita light image corresponding to said charge image. This critical range ofphosphor thicknesses will be discussed in greater detail with referenceto FlG. 4.

An electrical connection may be provided to the upper and lowerconductive films 102 and 103 from the exterior of the envelope by, forexample, extending the conductive films through the glass frit seal 18so that they may be contacted by the connector coatings 5l) and 51,respectively, on the exterior of such envelope. However, this connectionmay be made in any number of ways including the provision of aconductive coating of silver or other suitable material on the surfaceof face plate 1d and extending through the seal 13 into contact with thefilms. Other alternatives include making seal 1S of a conductive glassfrit, providing a metal pin through such seal, or using a connector plugsimilar to plugs 30, 34 and 38. One such connector plug 3d is shown inFIG. 3 as extending through an aperture in the funnel portion 12 andmaking contact with the third electrode wall coating 36. This plug is inthe form of a body of ceramic material 105 similar to the ceramicmaterial of funnel envelope portion 12 and provided with a connectorcoating 108 of silver or other conductive material on the exterior ofsuch plug body so that coating 108 provides an electrical connectionfrom the exterior of the envelope to the third electrode wall coating.The plug connector 38 is sealed in the aperture in the funnel portion 12of the envelope by means of a ceramic-to-metal seal 110.

The upper conductive film 102. is connected to a DC. target voltageproduced across a fixed resistor 11?, by current flowing to the groundedterminal of such resistor from a source of +500 volts through a variableresistor 12d. Thus, the setting of the variable resistor 121i controlsthe flood gun cathode to target voltage of the upper portion of thesplit screen storage target 26 so that this portion may be operated in astorage or a nou-storage mode by setting the target voltage above orbelow the retention threshold voltage and below the fade positivevoltage. ln addition, the variable resistor 12) may be employed to erasea charge image produced on the upper portion of the phosphor layer 1M bythe writing gun 17 and stored due to the secondary emission caused bythe flood electrons emitted by the flood gun 19 striking such phosphorlayer. The lower conductive film 1&13 is similarly connected to a D.C.target voltage produced across a fixed resistor 114 by current flowingthrough a variable resistor 116 whose setting `controls the operation ofthe lower portion of the storage target. Therefore, the two portions ofthe phosphor layer overlying the conductive film 102 and 103 may beoperated independently in a storage or non-storage mode so that signaldisplays may be viewed but not stored on one target portion and then maybe moved to the other target portion for storage by adjusting thevertical positioning control of the tube.

The direct viewing storage tube of the present invention may also beprovided wit-h electrical readout merely by using the writing gun 17also as a reading gun to scan the phosphor layer 104 of the storagetarget and produce electrical readout signals on the conductive films102 and 103 corresponding to any charge images stored on such phosphorlayer. In this regard, the common connection of resistors 114 and 116 isconnected to a rst output terminal 122 through a D.C. blocking capacitor124, and the common connection of resistors 118 and 120 is connected toa second output terminal 126 through a D.C. blocking capacitor 128.These output terminals 122 and 126 may be connected to the Z-axis inputof a remote television monitor tube (not shown) to vary t-he intensityof the electron beam in such monitor tube in accordance with theelectrical readout signals produced on conductive ilms 102 and 103. Thenif related television raster signals `are applied to the horizontal andvertical deflection plates of both the storage tube and the monitortube, the charge image stored on the storage target 26 will be displayedon the fluorescent screen of the monitor tube in a conventional manner.

The mesh electrode 40 may be supported by spot welding it to an annularspring channel member 130 after such channel member is inserted into anannular notch 132 in the inner surface of the ceramic funnel portion 12of the envelope. The channel member expands outwardly into engagementwith the bottom of the not-ch 132 and is held against longitudinalmovement by an annular ridge 134I extending from the inner surface ofthe funnel portion. A plurality of metal spring clips 135 may be securedto the channel member 132 by welding at spaced positions around suchchannel member so that such clips resiliently engage the second wallcoating 32 to electrically connect the mesh electrode to such wallcoating.

As in conventional bistable storage tubes, the electrons forming thewriting beam emitted from the writing gun 17 of the tube of the presentinvention have sufiicient velocity to cause secondary electrons to beemitted from the storage dielectric layer 104 of storage target 26. Theresult is that the area at the point of impact of such writing beamacquires a net positive voltage above the first cross-over voltage onthe secondary emission characteristic of such phosphor material. The oodelectrons emitted from flood guns 19 are substantially uniformlydistributed over the storage target 26 and, in a stable range, do nothave sufficient impact velocity to cause much secondary emission fromtarget areas which have not been struck by the writing beam. Thus, thesecondary emission ratio is less than one and the ood electrons tend tonegatively charge thoseV areas of phosphor storage layer 104 which havenot been struck by the writing beam until such negative charge issuicient to repel most of the ood electrons. This target potentialcorresponds to, or approaches, the first stable point which is approximately equal to the potential of the ood gun cathodes. However, theflood electrons are further accelerated by any positive charge areas onthe phosphor storage layer 104 which are present due to the secondaryemission caused by bombardment of the writing beam. This additionalacceleration provides the flood electrons with sufiicient impactvelocity to cause greater secondary emission at those target .areasalready having a positive charge :lue to the writing beam. Therefore,the secondary emission ratio is greater than one and the positive chargeof these areas is thereby increased tending to drive the targetpotential to a voltage corresponding to the second stable pointapproximately equal to the target voltage on the conductive films 102and 103. It should be noted that the secondary emission characteristicsof the storage target are different for the writing beam and the oodbeam since their cathodes are at different voltages.

Some typical operating values for the electrodes in the storage tube ofFIGS. 1 to 4 are a writing gun cathode voltage of 3,000 volts, a floodgun cathode voltage of Zero volt, a flood gun grid voltage of -lS voltsand a flood gun anode voltage of +200 volts. Anode resistance coating 44may be operated at +220 volts, the first electrode coating 28 at +200volts, the second electrode coating 32 at +300 volts, and the thirdelectrode coating 36 at +50 volts. The voltage of the conductive film102 of the storage target may vary considerably but is typically held at+200 volts. In addition, the total current of both ood gun cathodes maybe about l5 milliamperes while the typical total operating current ofthe conductive film 102 may be from about 2 to 9 milliamperes.

The storage and display characteristics of the storage target of thepresent invention as a function of the relative thickness of phosphorstorage layer 104 are shown in FIG. 4. Briefiy, these characteristicsare a stable range curve 136, a light image brightness curve 138 and alight image contrast curve 140. The ordinate of the stable range curveis target voltage stable range and such stable range curve 136 increasesfrom zero volt to a maximum of about to 120 volts at point 142 and thenreturns to Zero volt at the thickness TS which represents .a criticalthickness above which the phosphor layer 104 will no longer store anelectrical charge image for an indefinite controllable time. Theabsolute value of Ts is in the neighborhood of .001 to .003 inchdepending upon the type of phosphor used and is approximately one-halfto onethird of the thickness Tc of conventional cathode ray tubescreens. It should be noted that the storage target of the presentinvention has an extremely wide stable range of operating voltages up toabout volts. In this range the phosphor storage layer will store acharge image without spreading or blurring for an indefinite time so asto provide a bistable storage tube. The stable range of conventionalbistable storage tubes is ordinarily defined as that range of collectorvoltages between the retention threshold voltage where effectivebistable storage begins and the fade positive point voltage where suchstorage stops and the charge becomes uniform over the phosphor layer dueto the .action of the flood guns. In the present storage tube the stablerange voltages are those established between the cathodes of the floodguns 76 and the conductive lm 102. The 100 volt stable range referred tois the initial range for new tubes and this large range is important ina bistable storage tube because the width of the stable range reduceswith use due to deterioration of the storage dielectric caused byelectron bombardment so that the lifetime of the tube is determined to alarge extent by the width of its initial stable range.

It has been found that the brightness curve 138 increases from zerocontinuously `and at a rather rapid initial rate as the thickness of thephosphor layer 104 is increased from zero to TS so that adequate imagebrightness may be obtained with very thin layers far below thatcorresponding to the maximum stable range at point 142. However, thecontrast curve 140 increases in value from Zero at a much lower initialrate than the brightness curve 138 as the phosphor thickness isincreased from zero to Ts. Thus, the contrast curve usually determinesthe lower limit of useful phosphor thicknesses. While the absolutevalues of brightness and contrast are not indicated, the lower limitthickness at which curve 140 reaches a useful contrast is somewhere tothe left of the thickness corresponding to point 142 depending upon theuse to which the storage tubes are put. Thus, there is a critical rangeof thicknesses for the phosphor storage layer 104 beginning above Zeroand ending below Ts within which such a layer will store an electricalcharge image for an indefinite controllable time and still give adequatelight image brightness and contrast. It will be noted that thebrightness curve 138 levels off so that no substantial increase inbrightness is obtained by increasing the thickness of the phosphorbeyond the thickness Tc where the curve is substantially level. Abovethe thickness Tc, brightness decreases. Thus, the operative range ofphosphor thicknesses depends upon both the particular type of phosphoremployed and the intended use of the storage tube but, in general, thelower limit of this range is somewhat greater than zero and the upperlimit is somewhat below one-half the thickness Tc at which thebrightness curve becomes substantially level. For P-l type phosphor thiscritical range of thicknesses is approximately from .001 to .0025 inch.

It has been found that the contrast of the light image of the storagetube of the present invention can be er1- hanced by applying 35 voltpulses to the cathode of the flood gun 19 or +35 volt pulses to thetarget electrode film 102 for about 50% of the duty cycle of such floodgun. This reduces the background light emitted by the directviewingstorage target 26 since the more energetic electrons from the flood gunduring contrast enhancement tend to drive the surface of the storagelayer 104 from the +40 volts obtained by the ood beam electrons towardthe zero volts potential of the cathodes of the ood guns.

It has also been found that an improved type of operation of the storagetube results in increasing the stored writing speed of such tube by afactor on the order of two times for transient signals and one thousandtimes for repetitive signals. This increase in writing speed may beaccomplished by turning off the flood gun during writing, as discussedabove. Thus, the ood electrons are prevented from striking the phosphorlayer 104 during the writing action of the writing electron beam as itmoves across the surface of such phosphor layer to produce a chargeimage thereon, until after the voltage of such charge image exceeds therst cross-over voltage which is the minimum voltage necessary forstorage. One way of. accomplishing this is to apply a large negativepulse of about 200 volts to a control grid of the flood gun 19 or a +200volt pulse to the cathode of such flood gun during the time of suchwriting action. This pulse may be produced by triggering a suitablepulse generator with a portion of the input signal and transmitting theremainder of such input signal through a delay line before applying itto the vertical detiecti-on plates of the writing gun 17. Another way isto employ a switch 144 as shown in FIG. l, to change such control gridvoltage from to 200 volts to cut off the flood gun by moving the movablecontact of such switch from the viewing position shown labeled View tothe position labeled Integrate This enables fast rise time transient.signals applied to the vertical deflection plates of the writing gun 17to produce a stored charge image on the storage target which could notpreviously be stored when flood electrons were allowed to strike suchtarget during writing. As has been previously pointed out, the floodelectrons act to oppose writing because they tend to drive the potentialof the charge image down toward the voltage of the ood gun cathode inthose target areas which are initially charged below the rst cross-overvoltages. By turning off the ood gun during Writing this opposing actionis eliminated and the writing speed of the storage tube effectivelyincreased. The above technique can also be employed for storing thecharge image of an extremely high frequency repetitive signal bymaintaining the ood gun in its cut oit condition during severalsuccessive cycles of such signal. Thus, the voltages produced on thestorage target by the charge images of the successive wave forms of suchrepetitive signal are added together or integrated because such waveforms are superimposed on the same area of the phosphor layer of suchtarget. If the ood electrons are prevented from striking the phosphorstorage dielectric layer until after the total voltage of the chargeimage exceeds the first cross-over voltage, such charge image will bestored for an Vunlimited time when such flood electrons are subsequentlyallowed to bombard such phosphor dielectric layer. This charge imageintegration method of operating a storage tube applies to all secondaryemission storage tubes which employ low velocity electrons for holdingthe charge image, including those using transmission type storagetargets.

It has also been discovered that the writing speed and the erase speedof the storage tube may be increased by about 25% if the operating levelof the target electrode film 102 is shifted about ten volts positive andnegative of normal during writing and erasing, respectively. This isapparently due to the fact that writing speed increases while the erasespeed decreases with increased target voltage over the stable range ofoperating voltages. It should be noted that the mesh electrode 40isolates the writing gun from changes in target voltage so that thedeection sensitivity of such writing gun does not vary during thevoltage pulsing of the conductive film electrode 102 for contrastenhancement or for increased writing and erase speeds.

In addition, the storage tube of the present invention may be providedwith variable image persistence by lowering the potential on theconductive film electrode 102 to about +5 volts above the retentionthreshold voltage for the target and allowing the mesh electrode 40 tocollect most of the secondary electrons emitted from the phosphor layer104. In this manner the persistence time of the light image emitted bysuch phosphor layer can be adjusted so that the image of a previoussignal wave form is extinguished progressively immediately in front ofthe electron beam forming a subsequent signal wave form which is veryuseful in medical diagnosis and research. This is believed to be due tothe fact that the phosphor layer is not exactly uniform in thickness sothat the retention threshold varies in different areas and some of theseareas are not storing a bistable charge image. Halftone storage is alsopossible during these conditions.

From the above -it should be obvious to one having `ordinary skill inthe art that various changes may be made in the detail of the abovedescribed preferred embodiments of the present invention. Therefore, thescope of the present invention should only be determined by thefollowing claims.

I claim:

1. Apparatus for operating a bistable charge image storage device toenable a faster writing rate comprising:

a storage target including a storage dielectric;

rst means for forming a charge image on said storage dielectric of arepetitive input signal applied to said storage device;

second means for bombarding the storage dielectric substantiallyuniformly with low velocity electrons to enable bistable storage of saidcharge image by causing secondary electron emission from said storagedielectric; and

third means for preventing the low velocity electrons from bombardingthe storage dielectric during the formation of the charge image untilafter the potential of such charge image exceeds the minimum voltagenecessary for bistable storage by causing several successive cycles ofsaid input signal to lie superimposed on said storage dielectric toincrease the potential of said charge image above said minimum voltage.

2, Apparatus in accordance with claim 1 in which the first meansbombards the storage dielectric with high velocity electrons which aremodulated by the input signal to form such charge image.

3. Apparatus in accordance with claim 2 in which the storage device is acathode ray tube and the charge image is formed by deecting the beam ofhigh velocity electrons in accordance with the input signal applied tosaid tube so that such charge image is the waveform of said inputsignal.

4. Apparatus in accordance with claim 3 in which the storage dielectricis phosphor material which emits a light image corresponding to thestored charge image.

5. Apparatus in accordance with claim 1 in which the third means appliesa large reverse bias voltage between a ood gun cathode which emits thelow velocity electrons and a control grid associated with said ood guncathode to prevent said low velocity electrons from bombarding thestorage dielectric.

6. Apparatus in accordance with claim 5 in which the third meansincludes a puiser means for applying a reverse bias voltage pulsebetween the flood gun cathode and the control grid.

'1. Apparatus in accordance with claim 4 which also includes a fourthmeans for momentarily increasing the velocity of the low velocityelectrons for a portion of the time said low velocity electrons bombardthe storage dielectric to increase the contrast of the light image.

8. Apparatus in accordance with claim 7 in which the 25 fourth meansincludes pulser means for applying a plurality of voltage pulses to atarget electrode of the storage target to increase the velocity of thelow velocity electrons.

9. Apparatus in accordance with claim 4 in which the phosphor storagedielectric is a thin integral layer of phosphor material applied over alight transparent electrically conductive support surface, said layerbeing sutlciently porous to enable secondary electrons emitted from oneside of the layer by bombardment of the low velocity electrons to betransmitted through said layer and collected by said conductive surfaceon the opposite side of the layer.

References Cited UNITED STATES PATENTS 3,144,579 8/1964 Holsirlger315-12 X 3,155,869 11/1964 Firmin 315-12 3,165,665 1/1965 Firmin 315-123,214,516 10/1965 Anderson 315--12 X` 3,214,631 10/1965 Anderson 315--123,259,791 7/1966 Jensen 315--12v OTHER REFERENCES Knoil, M., and B.Kazan, Storage Tubes, John Wiley & Sons, New York, 1952, pp. 24, 53, 77.

Haeif, Andrew V.: A Memory Tube in Electronics, September 1947, pp.80-83.

DAVID G. REDlNBAUGI-I, Primary Examiner.

T. A. GALLAGHER, Assistant Examiner.

1. APPARATUS FOR OPERATING A BISTABLE CHARGE IMAGE STORAGE DEVICE TOENABLE A FASTER WRITING RATE COMPRISING: A STORAGE TARGET INCLUDING ASTORAGE DIELECTRIC; FIRST MEANS FOR FORMING A CHARGE IMAGE ON SAIDSTORAGE DIELECTRIC OF A REPETITIVE INPUT SIGNAL APPLIED TO SAID STORAGEDEVICE; SECOND MEANS FOR BOMBARDING THE STORAGE DIELECTRIC SUBSTANTIALLYUNIFORMLY WITH LOW VELOCITY ELECTRONS TO ENABLE BISTABLE STORAGE OF SAIDCHARGE IMAGE BY CAUSING SECONDARY ELECTRON EMISSION FROM SAID STORAGEDIELECTRIC; AND THIRD MEANS FOR PREVENTING THE LOW VELOCITY ELECTRONSFROM BOMBARDING THE STORAGE DIELECTRIC DURING THE FORMATION OF THECHARGE IMAGE UNTIL AFTER THE POTENTIAL OF SUCH CHARGE IMAGE EXCEEDS THEMINIMUM VOLTAGE NECESSARY FOR BISTABLE STORAGE BY CAUSING SEVERALSUCCESSIVE CYCLES OF SAID INPUT SIGNAL TO BE SUPERIMPOSED ON SAIDSTORAGE DIELECTRIC TO INCREASE THE POTENTIAL OF SAID CHARGE ABOVE SAIDMINIMUM VOLTAGE.